Lesson focus: Atoms contain a tiny positive nucleus of protonA tiny particle with a positive charge, found in the nucleus of an atom. and neutronA tiny particle with no charge, found in the nucleus of an atom. surrounded by electronA negatively charged particle with very little mass, found in shells around the nucleus of an atom.; isotopeAtoms of the same element with the same number of protons but different numbers of neutrons. share proton number but differ in neutron number.
Determine subatomic particle numbers and calculate relative atomic mass from isotopic abundance.
Build on earlier learning
AtomsYear 7 introduced atoms. This lesson develops the nuclear model, isotopes and weighted relative atomic mass.
You will be successful when you can…
- State particle charges and relative masses.
- Use atomic and mass numbers.
- Define isotope correctly.
- Calculate weighted Ar.
Words that carry the science
- proton
- Nuclear particle with relative charge +1 and relative mass 1.
- neutron
- Nuclear particle with charge 0 and relative mass 1.
- electron
- Particle with relative charge −1 and very small relative mass.
- atomic number
- Number of protons in an atom nucleus.
- mass number
- Total number of protons and neutrons in one isotope.
- isotope
- Atoms of one element with equal proton number but different neutron number.
The nucleus contains nearly all atomic mass
01The nucleus contains nearly all atomic mass
protonA tiny particle with a positive charge, found in the nucleus of an atom. and neutronA tiny particle with no charge, found in the nucleus of an atom. occupy a tiny nucleus. electronA negatively charged particle with very little mass, found in shells around the nucleus of an atom. occupy regions around it. Proton and neutron relative masses are about 1; electron mass is approximately 1/1836 of a proton and treated as negligible in mass-number calculations.
atomic numberThe number of protons in an atom; each element has its own atomic number. Z equals proton number. mass numberThe total number of protons and neutrons in an atom. A equals protons plus neutrons, so neutrons = A − Z. A neutral atom has equal proton and electron numbers.
02Isotopes are the same element
isotopeAtoms of the same element with the same number of protons but different numbers of neutrons. have the same proton number, so they are one element, but different neutron numbers. Their electron arrangements—and therefore chemical behaviour—are usually very similar.
Isotopes can differ in mass and nuclear stability. Mass number belongs to one isotope and is a whole number.
03Abundance produces a weighted mean
Calculate Ar = Σ(isotopic mass × abundance) ÷ Σabundance. Percentage abundances sum to 100, but the general denominator prevents hidden mistakes.
The resulting Ar can be non-integer because it averages different isotopes; no individual atom has the average neutron count.
Build an atom
The number of protons decides the element. Neutrons add mass but not charge; atoms of one element with different numbers of neutrons are isotopes. Electrons fill shells (2, then 8, then 8); if they do not match the protons, you have an ion.
Proton Neutron Electron
Controls
Calculate chlorine Ar
A sample is 75% chlorine-35 and 25% chlorine-37.
- Multiply masses by abundances: 35 × 75 and 37 × 25.
- Add weighted values: 2625 + 925 = 3550.
- Divide by total abundance: 3550 ÷ 100.
- Ar = 35.5.
How to make the reasoning visible: 35.5 is a weighted population mean, not the mass number of one chlorine atom.
Decode nuclear notation
Use proton and mass numbers to complete each particle account.
| Species | Protons | Neutrons | Electrons |
|---|---|---|---|
| ²³Na atom (Z=11) | 11 | 12 | 11 |
| ²⁴Mg²⁺ (Z=12) | 12 | 12 | 10 |
| ³⁵Cl⁻ (Z=17) | 17 | 18 | 18 |
| ³⁷Cl atom (Z=17) | 17 | 20 | 17 |
Why are chlorine-35 and chlorine-37 isotopes?
Which particle number changes when an ion forms?
Check your understanding
Answer from memory, then check against the suggested answer.
What is atomic number?
How is neutron number found?
Define isotope.
Element X has isotopes 10 (20%) and 11 (80%). Calculate Ar.
Why do isotopes react similarly but diffuse at slightly different rates?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Copper is 69.2% copper-63 and 30.8% copper-65.
Calculate the relative atomic mass of copper. Give your answer to 1 decimal place.
(4)Explain why the relative atomic mass is not a whole number.
(1)Total for this question: 5 marks
Command word: For calculateA command word: work out a number answer, showing your working and the unit., show the weighted products and denominator, then interpret the decimal.